1. Field of the Invention
The present invention relates to a mask placed between a sample and the ion gun of a sample preparation apparatus utilizing ion milling. The invention also relates to this sample preparation apparatus having the mask.
2. Description of Related Art
One known apparatus for preparing ion-milled samples on a scanning electron microscope (SEM) or transmission electron microscope (TEM) is described, for example, in Japanese Patent No. 3263920. This known apparatus is now described by referring to FIG. 1. The apparatus uses a mask in the form of a flat plate. This mask is made of a shielding material, has a straight edge, and is placed over a sample. The sample is etched by an ion beam directed at it with the boundary defined by the edge of the mask. The hatched portion in the figure is a portion of the sample to be etched to obtain a sample cross section S.
FIGS. 2(a)-2(d) show the process in which the sample is etched by the apparatus shown in FIG. 1. In the initial phase of the etching, see FIG. 2(a), the sample is etched from its one corner. At the same time, the mask irradiated with the ion beam is etched from its one corner in the same way as the sample.
When the corner of the mask is etched and becomes rounded in this way, ions hitting the rounded portion slide down along the side surface of the mask, as illustrated in FIG. 2(d), the “diagram illustrating ion path”. As a result, the ions travel longer distances (longer ion path lengths). Consequently, every ion etches the side surface of the mask over a longer distance. On the other hand, ions hitting the top surface of the mask are stopped there and thus the ion path length is quite small. As a result, the amount by which the rounded side surface of the mask is etched is considerably greater than the amount by which the top surface is etched.
Through the state shown in FIG. 2(b), the sample and mask are finally etched as shown in FIG. 2(c). Since the side surface of the mask has been etched considerably as shown in FIG. 2(c), the portions of the sample closer to the mask are overetched. That is, a portion of the sample including the desired cross section S is etched.